Cadherins expression during gamete maturation and fertilization in the rat

A role for adhesion molecules in gamete fusion, preceding fertilization, has been previously suggested. We investigated the presence of cadherins, Ca²⁺ dependent cell-cell adhesion molecules, in rat oocytes and spermatozoa using an antipan-cadherin antibody and specific antibodies against the 3 classical cadherins: E- (epithelial), P- (placental), and N- (neural) cadherins. Electrophoretic separation was performed on samples of lysed oocytes of different stages: germinal vesicle oocytes, metaphase II eggs, newly fertilized and 2-cell embryos, as well as spermatozoa from testes, caput and cauda epididymis and ejaculate. Localization of cadherins was determined on intact, gametes by immunocytochemistry, using confocal microscopy. Immunoblotting with the pan-cadherin antibody revealed a major band of ∼120 kD in all oocyte and sperm extracts. Oocytes presented E-cadherin at appropriate molecular weight but N-cadherin only as a specific 40 kD band. In sperm lysate, at all stages, both E- and N-cadherin were demonstrated as major protein bands but a series of lower molecular weight proteins (that may represent protein degradation) were also detected. Immunohistochemical evaluation showed that E- and N-cadherins are already present on the plasma membrane of immature unfertilized oocytes, although their concentration increases after fertilization in early cleavage stage embryos. Cadherin localization on spermatozoa changed during maturation from a dispersed pattern over the entire head plasma membrane of testicular spermatozoa to a restricted equatorial and post-acrosomal plasma membrane staining in ejaculated spermatozoa. These findings suggest a specific cadherin organization at the fusogenic domains of both gametes.